Hot-rolled ferritic stainless steel sheet with excellent cold cracking resistance and manufacturing process therefor

ABSTRACT

This hot-rolled ferritic stainless steel sheet contains, in terms of % by mass: 0.0150% or less of C, 0.01% to 2.00% of Si, 0.01% to 2.00% of Mn, less than 0.040% of P, 0.010% or less of S, 10.0% to 30.0% of Cr, 0.001% to 0.100% of Al, and 0.0200% or less of N, with a balance being Fe and unavoidable impurities, wherein in a cross section in a range of ¼ to ¾ of a sheet thickness, a length L of all crystal grain boundaries having orientation differences of 1° or more to less than 180° and a length La of subgrain boundaries having orientation differences of 1° or more to less than 15° satisfy a relation of La/L≧0.20.

TECHNICAL FIELD

The present invention relates to a hot-rolled ferritic stainless steel sheet with excellent cold cracking resistance and a manufacturing method therefor.

The present application claims priority on Japanese Patent Application No.

2011-270092 filed on Dec. 9, 2011, the content of which is incorporated herein by reference.

BACKGROUND ART

Ferritic stainless steel is used for a variety of applications such as household electric appliances, building materials, and automobile components. An appropriate amount of various elements are added to this steel in the related art according to required characteristics such as corrosion resistance, high temperature characteristics, and the like.

It is known that the addition of Cr, Mo, or Ni is effective for the purpose of improving corrosion resistance. Further, the addition of Nb, Al, or Si is effective for improving high temperature characteristics (strength and oxidation resistance).

In general, as the addition amount of these elements increase, the characteristics thereof are further improved; however, manufacturability, particularly, cold cracking resistance is degraded. For this reason, the upper limit of the addition amount is determined.

The term “cold cracking resistance” means cracks generated when the coil of a hot-rolled sheet (the hot-rolled sheet coiled in a coil shape) is uncoiled and then the hot-rolled sheet is passed through a continuous pickling line, a continuous annealing and pickling line, a cold rolling line, and the like, and it is considered that cracks are generated because of insufficient toughness of the hot-rolled sheet.

With regard to steels of the ferritic stainless steel containing many additive elements, cracks are easily generated in the winter season where the temperature is low.

Patent Document 1 and Patent Document 2 are well-known as solutions for improving toughness of hot-rolled sheets formed of ferritic stainless steel with a large amount of Cr and stainless steel to which Al is added.

Patent Document 1 discloses a technology of coiling a steel sheet at a temperature of 400° C. to 600° C. after completing finish hot rolling and then immediately rapid cooling at a cooling rate of equal to or more than that of water cooling, as a technology of improving the toughness of a hot-rolled sheet formed of steel to which Cr is added at a content in a range of 25% by weight to 35% by weight.

Patent Document 2 discloses a method of coiling a steel sheet at a coiling temperature of 550° C. to 650° C. to obtain a coiled steel strip and then immersing the coiled steel strip in a water tank after 3 hours or less is passed from the coiling of the steel sheet.

As described above, Patent Documents 1 and 2 disclose the technologies as technologies of improving the toughness of the hot-rolled steel. However, when the knowledge in the related art was applied to various ferritic stainless steels by the present inventors, there were cases that cold cracking was generated; and therefore, it was understood that the knowledge was not necessarily effective for the improvement of the toughness thereof. In other words, the technologies in the related art are not sufficiently effective and further improvement is required.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Unexamined Patent Application, First Publication No. H5-320764

Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2001-26826

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In light of the above problem, the present invention aims to provide a hot-rolled ferritic stainless steel sheet with excellent cold cracking resistance and a manufacturing method therefor.

Means for Solving the Problems

In order to solve the above-described problem, the present inventors inspected the relationship between the conditions of coiling a hot-rolled ferritic stainless steel sheet and the toughness of the hot-rolled steel.

First, ferritic stainless steels whose components were changed were hot-rolled down to a thickness of 5 mm in a laboratory to obtain hot-rolled steel sheets. Subsequently, each of the hot-rolled steel sheets was inserted into a furnace in which the temperature therein was controlled to be a coiling temperature; and thereby, a coiling treatment was simulated. The coiling temperature (temperature inside the furnace) was changed in a range of 550° C. to 950° C. and the time of the coiling treatment (heating in the furnace) was changed in a range of 0.1 h to 100 h. Next, the steel sheets were cooled down to room temperature by water cooling; and thereby, hot-rolled steel sheets were produced.

Each of the obtained hot-rolled steel sheets was subjected to a Charpy test and an impact value (toughness) at room temperature (25° C.) was evaluated.

In addition, the microstructure of each of the hot-rolled steel sheets manufactured under the above-described various conditions was inspected with an optical microscope and EBSP (Electronic Back Scattering Pattern analyzing method). The recrystallization state of the steel sheet was inspected with the optical microscope. Additionally, presence of a subgrain boundary in a crystal grain was inspected with the EBSP.

The measurement by the EBSP was performed by the method described in the following embodiment. Specifically, samples for measurement which had cross sections (L cross section) parallel to a rolling direction and perpendicular to a sheet surface direction were collected. The L cross section of the sample for measurement was subjected to grinding using electrolytic grinding or colloidal silica. In the L cross section, the range of ¼ t to ¾ t of the sheet thickness t (¼ to ¾ of the sheet thickness) was used as a measuring range. Among the measuring range, a crystal orientation was measured in a measuring step (pitch) of 0.2 μm in a range of 100 μm×100 μm. Determination of a crystal grain boundary and a subgrain boundary was performed as follows. An interface having an orientation difference of 1° or more to less than 180° between adjacent measurement points was regarded as a grain boundary. Among these, a grain boundary having an orientation difference of 1° or more to less than 15° was regarded as a subgrain boundary. The obtained knowledge is exemplified as follows.

(1) The Charpy impact value of the obtained hot-rolled steel sheet was largely changed in a range of 5 J/cm² to approximately 100 J/cm² depending on manufacturing conditions.

(2) When the microstructure of the obtained hot-rolled steel sheet was observed with an optical microscope, three structures were recognized which were an unrecrystallized structure, a complete recrystallized structure, and a mixed structure of unrecrystallized structure and recrystallized structure are recognized In the case of the complete recrystallized structure, the Charpy impact value of the hot-rolled steel sheet was in a range of less than 20 J/cm². In the case of the unrecrystallized grain and in the case of the mixed structure of unrecrystallized structure and recrystallized structure, it was recognized that the Charpy impact value was in a range of 20 J/cm² or more in some cases.

(3) The total (length L of all crystal grain boundaries) of the lengths of the crystal grain boundaries having orientation differences of 1° or more to less than 180° and the total (length La of subgrain boundaries) of the lengths of subgrain boundaries having orientation differences of 1° or more to less than 15° were obtained through the inspection of crystal grain boundaries using EBSP. Then, a relationship between the ratio of La/L and the Charpy impact value was obtained.

FIG. 1 illustrates the relationship of a toughness value (Charpy impact value) and the ratio of La/L when the coiling conditions (temperature and time) in various ferritic stainless steels were changed. As shown in FIG. 1, the Charpy impact value is in a range of 20 J/cm² or more which is high when the ratio of La/L is in a range of 0.20 or more, and the Charpy impact value is in a range of less than 20 J/cm² when the ratio of La/L is in a range of less than 0.20.

In general, the crystal grain boundary indicates an orientation difference between adjacent crystal grains. In the case of the complete recrystallized structure, substantially all the crystal grains at both sides interposing the crystal grain boundary have orientation differences of 15° or more. That is, crystal grain boundaries having orientation differences of 1° or more to less than 15° are not present in the complete recrystallized structure; and therefore, the ratio of La/L becomes closer to 0.

In the present test, in the case where the coiling temperature was 900° C., the complete recrystallized structure was obtained in any type of steels, and the Charpy impact value was in a range of less than 20 J/cm² in any cases. On the other hand, in the case where the coiling temperature was in a range of 800° C. or less and the Charpy impact value was in a range of 20 J/cm² or more, unrecrystallized crystal grains appeared to be largely present in the microstructure (optical microscope structure), and subgrain boundaries were largely present observed by the analysis of EBSP.

The present invention can be obtained based on this knowledge and features of one aspect of the present invention are as follows.

(1) A hot-rolled ferritic stainless steel sheet with excellent cold cracking resistance contains, in terms of % by mass: 0.0150% or less of C; 0.01% to 2.00% of Si; 0.01% to 2.00% of Mn; less than 0.040% of P; 0.010% or less of S; 10.0% to 30.0% of Cr; 0.001% to 3.00% of Al; and 0.0200% or less of N, with a balance being Fe and unavoidable impurities, wherein in a cross section in a range of ¼ to ¾ of a sheet thickness, a length L of all crystal grain boundaries having orientation differences of 1° or more to less than 180° and a length La of subgrain boundaries having orientation differences of 1° or more to less than 15° satisfy a relation of La/L≧0.20.

(2) The hot-rolled ferritic stainless steel sheet with excellent cold cracking resistance according to (1), further contains one or more selected from, in terms of % by mass: 0.05% to 0.70% of Nb; 0.05% to 0.30% of Ti; 0.1% to 2.5% of Mo; 0.1% to 1.5% of Ni; 0.0001% to 0.0025% of B; 0.1% to 2.0% of Cu; and 0.03% to 0.35% of Sn, wherein in a case where either one or both of Nb and Ti are included, the following formula (1) is satisfied.

Nb/93+Ti/48≧C/12+N/14   (1)

Element symbols in the formula (1) indicate contents of the respective elements in terms of % by mass.

(3) The hot-rolled ferritic stainless steel sheet with excellent cold cracking resistance according to (1) or (2), wherein the content of Al is in a range of more than 0.10% to 3.00%.

(4) A method for manufacturing the hot-rolled ferritic stainless steel sheet with excellent cold cracking resistance according to any one of (1) to (3), includes: casting ferritic stainless steel having the steel composition according to any one of (1) to (3) to generate a semi-finished product and subjecting the semi-finished product to hot rolling under a condition where a finishing temperature is in a range of 800° C. to 1000° C. to generate a hot-rolled steel sheet; subsequently, coiling the hot-rolled steel sheet in a coil shape at a temperature of more than 650° C. to 800° C.; and immersing the hot-rolled sheet coiled in a coil shape in a water tank within 1 hour after the coiling, maintaining the hot-rolled sheet in the water tank for 1 hour or more, and taking out the hot-rolled steel sheet.

Effects of the Invention

As described above, according to one aspect of the present invention, cold cracking resistance of a hot-rolled steel sheet can be prevented by increasing the ratio of subgrain boundaries influencing the toughness of a hot-rolled ferritic stainless steel sheet which contains various elements.

Further, according to the hot-rolled ferritic stainless steel sheet of one aspect of the present invention, cold cracking is not generated even when continuous annealing or a pickling step is performed after hot rolling.

Furthermore, according to one aspect of the present invention, manufacturing yield can be increased or production efficiency can be improved by suppressing the cold cracking of various hot-rolled ferritic stainless steel sheets. As a result, effects extremely useful for industries in terms of reduction in manufacturing costs can be exhibited. In addition, energy usage can be suppressed by the improvement of production efficiency, and this contributes to global environmental conservation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relationship between a ratio (La/L) of a length La of subgrain boundaries having orientation differences of 1° or more to less than 15° to a length L of all crystal grain boundaries of a hot-rolled ferritic stainless steel sheet in the present embodiment and a Charpy impact value.

EMBODIMENTS OF THE INVENTION

Hereinafter, a hot-rolled ferritic stainless steel sheet of the present embodiment will be described in detail.

The hot-rolled ferritic stainless steel sheet of the present embodiment has a steel composition which contains, in terms of % by mass: 0.0150% or less of C; 0.01% to 2.00% of Si; 0.01% to 2.00% of Mn; less than 0.040% of P; 0.010% or less of S; 10.0% to 30.0% of Cr; 0.001% to 3.00% of Al; and 0.0200% or less of N with a balance being Fe and unavoidable impurities, wherein in a cross section in a range of ¼ to ¾ of a sheet thickness, a length L of all crystal grain boundaries having orientation differences of 1° or more to less than 180° and a length La of subgrain boundaries having orientation differences of 1° or more to less than 15° satisfy a relation of La/L≧0.20.

Hereinafter, the reason for restricting the steel composition of the hot-rolled steel sheet of the present embodiment will be described. In addition, the term “%” in regard to compositions means “% by mass” unless otherwise noted.

C: 0.0150% or less

When C is present in a solid solution state, grain-boundary corrosion resistance of a welded portion is degraded; and therefore, a large amount of C is not preferable. The upper limit of the amount of C is set to 0.0150%. In addition, the refining time and the manufacturing costs become increased when the amount of C is reduced not to influence the grain-boundary corrosion resistance. Therefore, the lower limit of the amount of C is preferably set to 0.0010%. Further, when considered from viewpoints of the grain-boundary corrosion resistance of a welded portion and the manufacturing costs, it is preferable that the amount of C be set to be in a range of 0.0020% to 0.0070%.

Si: 0.01% to 2.00%

Si is an element which improves oxidation resistance. However, workability of a product is degraded when a large amount of Si is added; and therefore, the upper limit of the amount of Si is set to 2.00%. On the other hand, since Si is inevitably mixed as a deoxidizing agent, the lower limit of the amount of Si is set to 0.01%. In addition, the amount of Si is preferably in a range of 0.02% to 0.97%.

Mn: 0.01% to 2.00%

Mn is an element that improves high temperature strength and oxidation resistance, however, the addition of a large amount of Mn results in degradation of workability of a product as is the case with Si. Accordingly, the upper limit of the amount of Mn is set to 2.00%. Further, since there are cases in which Mn is inevitably mixed, the lower limit of the amount of Mn is set to 0.01%. Furthermore, the amount of Mn is preferably in a range of 0.02% to 1.95%.

P: less than 0.040%

Since P is inevitably mixed from raw materials of Cr and the like, there are many cases in which 0.005% or more of P is mixed, however, P degrades ductility and manufacturability. Accordingly, it is preferable that the amount of P be as small as possible. However, since it is extremely difficult to perform excessive dephosphorization and manufacturing costs increase, the amount of P is set to be in a range of less than 0.04%.

S: 0.010% or less

Since there are cases in which S generates an easily soluble compound and degrades corrosion resistance, it is preferable that the amount of S be small; and therefore, the amount of S is set to be in a range of 0.010% or less. Further, since it is preferable that the amount of S be small from a viewpoint of corrosion resistance, the amount of S is preferably in a range of less than 0.0050%. It is more preferable that the lower limit of the amount of S be set to 0.0001% because a desulfurization technology is improved in recent years. It is still more preferable that the lower limit of the amount of S be set to 0.0005% when stable manufacturability is considered.

Cr: 10.0% to 30.0%

Cr is a basic element necessary for securing corrosion resistance, high temperature strength, and oxidation resistance, and 10.0% or more of Cr is necessarily added for exhibiting the effects thereof On the other hand, since the toughness is degraded when a large amount of Cr is added, the upper limit of the amount of Cr is set to 30.0%. In addition, when the amount of Cr is larger, the strength of the structure becomes higher, and embrittlement, which is referred to as “475° C. embrittlement”, specific to a steel containing a large amount of Cr tends to occur. Therefore, the amount of Cr is preferably in a range of 20.0% or less.

Al: 0.001% to 3.00%

Since Al is applied as a deoxidizing element, an appropriate amount of Al is added. When less than 0.001% of Al is added, deoxidizing ability becomes insufficient; and therefore, the lower limit thereof is set to 0.001%. On the other hand, the oxygen amount can be sufficiently reduced with 0.100% of Al, and the deoxidizing ability is substantially saturated with an addition amount of Al exceeding 0.100%. Therefore, the upper limit of the amount of Al is preferably 0.100% in a case of adding Al only for the purpose of deoxidization. In this case, the amount of Al is preferably in a range of 0.002% to 0.095%.

Further, Al has an effect of improving high temperature strength and corrosion resistance. In a case of adding Al for the purpose of improving high temperature strength and corrosion resistance, the amount of Al is preferably in a range of more than 0.10% to 3.00% and more preferably in a range of 0.50% to 2.00%. Furthermore, since the workability of a product is degraded when a large amount of Al is added, the upper limit of the amount of Al is set to 3.00%. The upper limit of the amount of Al is preferably 2.00% or less.

N: 0.0200% or less

As is the case with C, grain-boundary corrosion resistance of a welded portion is degraded when N is present in a solid solution state; and therefore, a large amount of N is not preferable. Accordingly, the upper limit of the amount of N is set to 0.0200%. In addition, the refining time and the manufacturing costs become increased so as to reduce the amount of N. Therefore, the lower limit of the amount of N is preferably set to 0.0030%. Further, when considered from viewpoints of the grain-boundary corrosion resistance of a welded portion and the manufacturing costs, it is preferable that the amount of N be set to be in a range of 0.0050% to 0.0120%.

Moreover, in the present embodiment, in addition to the above-described elements, it is preferable that either one or both of 0.05% to 0.70% of Nb and 0.05% to 0.30% of Ti be included to satisfy the following formula (1).

Nb/93+Ti/48≧C/12+N/14   (1)

Element symbols in the formula (1) indicate contents of the respective elements in terms of % by mass.

Nb and Ti generate precipitates together with C and N; and thereby, Nb and Ti reduce amounts of solid-solubilized C and N. In addition, when Nb and Ti are present in a solid solution state, the high temperature strength and thermal fatigue characteristics of a member are improved due to solid solution strengthening at a high temperature. In the case where Nb is contained, 0.05% or more of Nb is needed to be contained for fixing C and N, and it is preferable that 0.10% or more of Nb be contained. Further, in the case where Ti is contained, 0.05% or more of Ti is needed to be contained for fixing C and N.

In addition, the formula (1) described above is needed to be stoichiometrically satisfied for make all of C and N present in the steel be in a precipitation state.

On the other hand, when a large amount of Ti is exceedingly added, the toughness in the middle of manufacturing processes is degraded, and the generation of surface defects becomes remarkable in some cases. Therefore, the upper limit of Ti is set to 0.30%.

Further, addition of a large amount of Nb leads to the degradation of workability of a product. Accordingly, the upper limit of Nb is set to 0.70%, and it is more preferable to set the upper limit thereof to 0.55% or less.

Moreover, in the present embodiment, it is preferable to include one or more selected from 0.1% to 2.5% of Mo, 0.1% to 1.5% of Ni, 0.0001% to 0.0025% of B, 0.1% to 2.0% of Cu, and 0.03% to 0.35% of Sn in addition to the above-described elements. Mo, Ni, Cu, and Sn are elements that improve the high temperature strength or corrosion resistance and may be added as needed. Further, Ni has an effect of improving the toughness.

Since the improvement of the high temperature strength becomes remarkable when any one of Mo: 0.1% or more, Ni: 0.1% or more, Cu: 0.1% or more, and Sn: 0.03% or more is included, those amounts are set as lower limits thereof. It is more preferable that any one of Mo: 0.3% or more, Ni: 0.25% or more, Cu: 0.4% or more, and Sn: 0.10% or more be included for further improving the high temperature strength and the corrosion resistance.

Addition of large amounts of Mo, Ni, and Cu leads to degradation of picking properties; and thereby, productivity is degraded. Therefore, the upper limits of Mo, Ni, and Cu are respectively set to Mo: 2.5%, Ni: 1.5%, Cu: 2.0%, and more preferably set to Mo: 2.2% or less, Ni: 1.2% or less, and Cu: 1.4% or less. Since addition of a large amount of Sn leads to degradation in toughness and generation of surface defects, the upper limit of Sn is set to 0.35% and it is more preferable to set the upper limit thereof to be in a range of 0.20% or less.

B is an element which improves secondary workability. When a steel is used for the purpose in which the secondary workability is needed, B may be added when necessary. Since an effect of improving the secondary workability is exhibited when the addition amount of B is in a range of 0.0001% or more, this value is set to the lower limit and it is more preferable to set the lower limit thereof to be in a range of 0.0003% or more. Further, since there are cases in which the addition of a large amount of B leads to the degradation of toughness in the hot-rolled sheet and workability, the upper limit of B is set to 0.0025%, and it is more preferable to set the upper limit thereof to be in a range of 0.0015% or less.

In addition, as an important characteristic of the present embodiment, in a cross section in a range of 1/4 to 3/4 of the sheet thickness, the ratio between a length L of all crystal grain boundaries having orientation differences of 1° or more to less than 180° and a length La of subgrain boundaries having orientation differences of 1° or more to less than 15° satisfies a relation of La/L≧0.20.

The ratio between the length L of all crystal grain boundaries and the length La of subgrain boundaries is measured by the following method. First, samples for measurement are collected from 10 arbitrary portions of a hot-rolled steel sheet. The positions of the collected portions are not particularly limited. However, in practice, there are cases where a difference in coiling temperature is caused between a coiling-started part (top portion) and a coiling-finished part (bottom portion) when the hot-rolled steel sheet is actually coiled in a coil shape. Accordingly, in such a case, it is desired to collect samples for measurement from the top portion, the middle portion, and the bottom portion of the hot-rolled steel sheet in order to obtain an average value of the entire steel sheet. In regard to the width direction of the hot-rolled steel sheet, it is desired to collect samples for measurement from the approximate middle portion. Further, the samples for measurement are collected such that the samples have cross sections (L cross sections) parallel to the rolling direction and perpendicular to the sheet surface direction.

The L cross sections of the samples for measurement are subjected to grinding using electrolytic grinding or colloidal silica.

At or in the vicinity of the surface layer, relatively fine crystal grains tend to be easily generated and the toughness thereof is excellent in some cases. Accordingly, the vicinity of the center of the sheet thickness t, that is, the range of ¼ t to ¾ t of the sheet thickness t in the L cross section is used as a measuring range.

Next, the length of the crystal grain boundaries is measured using the EBSP by the following method. In the above-described measuring range, a crystal orientation is measured at a measuring step (pitch) of 0.2 μm in a range of 100 μm×100 μm. An interface having an orientation difference of 1° or more to less than 180° between adjacent measurement points is regarded as a grain boundary. Among these, a grain boundary having an orientation difference of 1° or more to less than 15° is regarded as a subgrain boundary.

The total of the lengths of all the crystal grain boundaries is calculated as “the length L of all crystal grain boundaries,” and the total of the lengths of subgrain boundaries is calculated as “the length La of subgrain boundaries.” Further, the ratio of La/L is obtained.

With regard to each of 10 samples for measurement, the ratio of La/L is obtained in the same way, and the average value of 10 values of La/L is calculated.

In the case where the ratio of La to L is less than 0.20, the toughness of the hot-rolled steel sheet becomes degraded to less than 20 J/cm²; and therefore, it is necessary to set the ratio of La/L to be in a range of 0.20 or more. As shown in FIG. 1, when the ratio of the subgrain boundaries is higher, the toughness of the hot-rolled steel sheet tends to be higher. Accordingly, it is preferable that the ratio of La/L be in a range of 0.35 or more. The upper limit of the ratio of La/L is not particularly limited, but the ratio of La/L becomes 1 when the orientation differences of all the grain boundaries are in a range of 1° or more to less than 15°. In the test performed by the present inventors, the ratio of La/L of 0.80 or more is not obtained.

Next, the method for manufacturing a hot-rolled ferritic stainless steel sheet in the present embodiment will be described.

The method for manufacturing a hot-rolled ferritic stainless steel sheet in the present embodiment has the following steps.

(1) A step of casting ferritic stainless steel having the above-described steel composition to generate a semi-finished product and then, subjecting the semi-finished product to hot rolling under a condition where a finishing temperature is in a range of 800° C. to 1000° C. to generate a hot-rolled steel sheet.

(2) Subsequent to the hot rolling, a step of coiling the hot-rolled steel sheet in a coil shape at a temperature of more than 650° C. to 800° C.

(3) A step of immersing the hot-rolled steel sheet coiled in a coil shape in a water tank within 1 hour after the coiling, maintaining the hot-rolled sheet in the water tank for 1 hour or more, and taking out the hot-rolled steel sheet.

Hereinafter, the method for manufacturing a hot-rolled ferritic stainless steel sheet in the present embodiment will be described in detail.

First, a ferritic stainless steel having the above-described steel composition is cast to generate a semi-finished product, and the semi-finished product is subjected to hot rolling to generate a hot-rolled steel sheet. Subsequently, the hot-rolled steel sheet to which the hot rolling (finish rolling) is applied is cooled down to the coiling temperature by water cooling, and the hot-rolled steel sheet is coiled in a coil shape at a coiling temperature. In the present embodiment, the finishing temperature of the hot rolling is set to be in a range of 800° C. to 1000° C., and the coiling temperature thereof is set to be in a range of more than 650° C. to 800° C.

In the case where the finishing temperature is less than 800° C. or more than 1000° C., it becomes extremely difficult to generate crystal grain boundaries having orientation differences of 1° or more to less than 15° after the coiling. Therefore, 800° C. and 1000° C. are set to the lower limit and the upper limit, respectively

Further, in the present embodiment, it is preferable not to generate an austenite phase at the time of the hot rolling. Whether the austenite phase is generated at the time of the hot rolling or not is determined depending on the amount of austenite generating elements in the steel, particularly, the amounts of C and N having a high austenite generating ability. In the hot-rolled steel sheet of the present embodiment, the amounts of both of C and N are small, and the generation of the austenite phase at the time of hot rolling is not recognized.

It is also difficult to generate crystal grain boundaries having orientation differences of 1° or more to less than 15° even in the case where the coiling temperature is in a range of 650° C. or less. In the case where the coiling temperature is in a range of more than 800° C., the recrystallization at the time of coiling progresses, and the ratio of the crystal grain boundaries having orientation differences of 15° or more to less than 180° increases; and therefore, the toughness is degraded.

Next, the hot-rolled steel sheet coiled in a coil shape is immersed in a water tank. This is because it is necessary to suppress the generation of precipitates that degrade the toughness in a slow cooling step after the coiling. Here, after the temperature of the hot-rolled steel sheet reaches the coiling temperature by water cooling subsequent to the finish rolling, a process in which the precipitates are generated and becomes coarsened strongly depends on the temperature and the elapsed time of the steel sheet after the coiling. In addition, in the case where the hot rolling is performed under general conditions and the coiling is performed at a coiling temperature of more than 650° C. to 800° C., the time from the end of the hot rolling to the moment where the temperature of the hot-rolled steel sheet reaches the coiling temperature is within 1 minute, and the cooling rate during that time is in a range of 3° C./sec or more. Under such a condition of the cooling rate, precipitates influencing the toughness are not generated during the time period from the completion of the finish rolling to the start of the coiling.

In regard to the generation of the precipitates that degrade the toughness, the time during the steel is maintained at the above-described coiling temperature becomes an important factor. In the present embodiment, it is necessary to immerse the hot-rolled steel sheet in a water tank within 1 hour after the coiling. In the case where the time taken from the completion of the coiling to the immersion in a water tank exceeds 1 hour, precipitates are generated during that time, and the toughness is degraded by the generated precipitates in some cases.

Further, the time for maintaining the hot-rolled steel sheet in a water tank after immersing the hot-rolled steel sheet in a water tank is also an important factor. It is preferable that the immersing time for maintaining the hot-rolled steel sheet in a water tank be in a range of 1 hour or more according to the present embodiment.

In the case where the immersing time of the hot-rolled steel sheet in a water tank is in a range of less than 1 hour which is short, the cooling becomes insufficient, and precipitates that degrade the toughness of the hot-rolled steel sheet are generated due to recuperation thereafter in some cases.

According to the hot-rolled ferritic stainless steel sheet of the present embodiment described above, a microstructure influencing the toughness of the hot-rolled steel sheet can be controlled by the requirements according to the components and the crystal grain boundaries; and as a result, cold cracking of the hot-rolled steel sheet can be prevented.

Further, according to the hot-rolled ferritic stainless steel sheet of the present embodiment, the cold cracking is not generated even when continuous annealing or a pickling step is carried out after hot rolling.

Furthermore, according to the hot-rolled ferritic stainless steel sheet of the present embodiment, since the cold cracking can be suppressed, the manufacturing yield can be increased and the production efficiency can be improved. As a result, effects extremely useful for industries in terms of reduction in manufacturing costs can be exhibited. In addition, energy usage in a manufacturing step can be suppressed due to improvement of production efficiency, and this contributes to global environmental conservation.

EXAMPLES

Hereinafter, effects of the present embodiment will be described with reference to the examples, but the present embodiment is not limited to the conditions used for the examples described below.

In the present example, at first, steels having the respective compositions listed in Table 1 were melted and cast to obtain steel ingots (semi-finished products).

Each of the steel ingots was grinded to a thickness of 90 mm. Each of the steel ingots was subjected to hot rolling at a finish temperature (FT) listed in Tables 2 and 3, and the steel ingot was rolled to a thickness of 5 mm to generate a hot-rolled steel sheet. Subsequently, each of the hot-rolled steel sheets was cooled down to the coiling temperature (CT) listed in Tables 2 and 3 by water cooling while the steel sheet temperature after rolling was monitored using a radiation thermometer. Here, the cooling rate at this time was 20° C./sec.

Next, each of the hot-rolled steel sheets was inserted into a furnace in which the temperature therein was controlled by the coiling temperature (CT) listed in Tables 2 and 3; and thereby, a coiling treatment was simulated. Subsequently, each of the hot-rolled steel sheets was immersed in a water tank after the elapsed time (t) listed in Tables 2 and 3 passed. Next, each of the hot-rolled steel sheet was maintained for the immersion time (tx) listed in Tables 2 and 3, and then the hot-rolled steel sheet was taken out.

Each of the obtained hot-rolled steel sheets had a complete ferrite single-phase structure.

In addition, characteristics (ratio La/L of the length La of the subgrain boundaries to the length L of all crystal grain boundaries) of the crystal grain boundaries were calculated using the EBSP in the same manner as that of the measurement method described in the embodiment.

Subsize Charpy impact test pieces were collected from each of the hot-rolled steel sheets in conformity to JIS Z 2202, and metal materials were subjected to the impact test in conformity with JIS Z 2242 by setting an impact direction to a direction perpendicular to the rolling direction. Impact absorbing energy was inspected under a condition where the test temperature was set to 25° C.

In addition, according to the obtained results, the cold cracking resistance (toughness) of the hot-rolled steel sheet was evaluated by the following method.

According to the present example, in a hot-rolled steel sheet having a Charpy impact value of less than 20 J/cm², the cold cracking was generated in a continuous annealing or a pickling step which was a step thereafter, and the yield was degraded. On the contrary, in a hot-rolled steel sheet having a Charpy impact value of 20 J/cm² or more, such cold cracking was not generated. Accordingly, the cold cracking resistance of the hot-rolled steel sheet having a Charpy impact value of less than 20 J/cm² was evaluated as “poor” and the cold cracking resistance of the hot-rolled steel sheet having a Charpy impact value of 20 J/cm² or more was evaluated as “excellent”. The Charpy impact values of less than 20 J/cm² were underlined in Tables 2 and 3.

The above-described manufacturing conditions and the evaluation results are listed in Tables 2 and 3.

Further, in Tables 2 and 3, FT represents the finishing temperature (° C.) of hot rolling and CT represents the coiling temperature (° C.) of the hot-rolled steel sheet. t represents the time (h) that was taken from the completion of the coiling to the start of water cooling (start of immersion), and tx represents the time (h) that was taken from the start of the water cooling to the completion thereof (from the start of immersion to the moment where the hot-rolled steel sheet was taken out).

Further, in Tables 1 to 3, the numerical values outside the ranges designated by the present embodiment were underlined.

TABLE 1 Steel C Si Mn P S Cr Al N Nb Ti Mo Ni Cu Sn B A 0.0011 0.15 0.12 0.023 0.001 22.1 0.010 0.0016 — — — — — — — B 0.0029 0.43 0.31 0.033 0.003 10.9 0.004 0.0060 — 0.15 — — — — — C 0.0035 0.10 1.50 0.014 0.005 19.0 0.003 0.0160 0.11 0.10 0.8  1.11 — — 0.0006 D 0.0110 1.22 0.75 0.029 0.006 24.3 0.077 0.0095 0.55 — — — — — 0.0024 E 0.0045 0.40 0.35 0.031 0.002 19.2 0.031 0.0119 0.21 0.15 2.10 — — — — F 0.0080 0.21 1.02 0.025 0.001 17.0 0.030 0.0130 0.53 0.12 0.31 0.10 1.25 — 0.0008 G 0.0032 0.15 0.12 0.023 0.001 22.1 0.010 0.0016 0.22 0.06 0.81 — — — 0.0009 H 0.0124 0.51 0.13 0.028 0.002 19.2 0.025 0.0180 0.38 — — 0.29 0.44 — — I 0.0034 0.12 0.24 0.017 0.007 14.5 0.074 0.0061 0.11 0.10 — — — 0.10 — J 0.0058 0.52 0.46 0.025 0.001 13.9 0.031 0.0080 — 0.14 — — 1.17 — — K 0.0081 0.95 0.22 0.022 0.002 27.1 0.006 0.0099 — 0.30 0.48 — — 0.34 0.0005 L 0.0115 0.25 0.33 0.028 0.001 32.5 0.007 0.0144 — 0.33 — — — — 0.0042 M 0.0045 0.45 0.22 0.033 0.003 21.4 1.85 0.0151 0.11 0.25 — — — — — N 0.0028 0.08 0.11 0.020 0.001 14.3 0.033 0.0089 0.13 0.08 — — — 0.03 0.0005 O 0.0033 0.25 0.09 0.028 0.001 13.5 0.025 0.0091 — 0.07 — — — 0.03 —

TABLE 2 Charpy impact FT CT t tx values Examples Steel (° C.) (° C.) (h) (h) La/L (J/cm²) 1 A 920 675 0.4 1.5 0.51 72 Example of the present invention 2 A 780 660 0.4 3.5 0.16 15 Comparative Example 3 A 880 500 0.7 2.4 0.02  9 Comparative Example 4 B 820 710 0.3 2.2 0.46 82 Example of the present invention 5 B 840 660 3.5 1.5 0.06  8 Comparative Example 6 B 1050  750 0.3 0.1 0.08 11 Comparative Example 7 C 850 760 0.5 11.0  0.34 65 Example of the present invention 8 C 910 780 1.2 0.4 0.04 12 Comparative Example 9 C 890 620 0.4 1.5 0.05 12 Comparative Example 10 C 840 500 2.0 2.6 0.11  5 Comparative Example 11 D 890 670 0.3 4.2 0.45 64 Example of the present invention 12 D 925 600 0.5 0.3 0.02 14 Comparative Example 13 E 900 730 0.4 1.4 0.39 43 Example of the present invention 14 E 890 600 0.7 3.2 0.02  5 Comparative Example 15 E 910 690 2.5 0.2 0.07  3 Comparative Example 16 F 970 750 0.4 3.0 0.47 65 Example of the present invention 17 F 960 840 0.9 5.0 0.04 10 Comparative Example 18 F 1050  790 0.4 4.5 0.03  9 Comparative Example 19 G 920 780 0.3 4.0 0.37 65 Example of the present invention 20 G 890 670 0.8 5.0 0.52 72 Example of the present invention 21 G 910 450 0.1 0.5 0.07 10 Comparative Example 22 H 870 660 0.1 3.5 0.29 80 Example of the present invention 23 H 1040  814 0.4 2.5 0.08 10 Comparative Example 24 H 830 550 0.3 1.4 0.07  9 Comparative Example 25 H 865 680 3.5 0.3 0.05 14 Comparative Example

TABLE 3 Charpy impact FT CT t tx values Example Steel (° C.) (° C.) (h) (h) La/L (J/cm²) 26 I 900 355 0.5 3.0 0.07 11 Comparative Example 27 I 910 685 0.3 5.0 0.45 72 Example of the present invention 28 I 770 660 0.2 3.0 0.18 18 Comparative Example 29 J 911 670 0.2 5.0 0.65 55 Example of the present invention 30 J 890 740 0.3 3.0 0.44 67 Example of the present invention 31 J 980 842 0.5 1.1 0.02 15 Comparative Example 32 K 925 721 0.4 2.4 0.34 56 Example of the present invention 33 K 830 560 3.5 7.5 0.04 17 Comparative Example 34 K 924 678 0.6 0.2 0.14  6 Comparative Example 35 L 820 731 0.4 4.5 0.11  5 Comparative Example 36 L 900 445 0.3 10.5  0.05  4 Comparative Example 37 L 860 695 0.4 4.3 0.09  3 Comparative Example 38 M 895 660 0.3 2.5 0.49 71 Example of the present invention 39 N 940 740 0.5 3.0 0.47 71 Example of the present invention 40 N 900 500 0.5 3.0 0.04  7 Comparative Example 41 N 1030  850 0.9 3.0 0.03  9 Comparative Example 42 O 830 780 0.2 2.6 0.42 66 Example of the present invention 43 O 950 700 2.0 2.2 0.06  8 Comparative Example 44 O 950 480 0.1 1.5 0.03  8 Comparative Example

As is clear from Tables 2 and 3, a hot-rolled ferritic stainless steel sheet with a Charpy impact value of 20 J/cm² or more and excellent cold cracking resistance, that is, a hot-rolled steel sheet with excellent toughness can be obtained in the examples of the present invention according to the present embodiment.

On the other hand, in the comparative examples outside the ranges designated by the present embodiment, the Charpy impact values were low in any cases. From this result, it is understood that the cold cracking resistance (toughness) of the hot-rolled steel sheets in the comparative examples was degraded.

From this result, the above-described knowledge can be verified and the grounds for limiting the above-described respective steel compositions and the constitutions can be backed up.

INDUSTRIAL APPLICABILITY

The hot-rolled ferritic stainless steel sheet of the present embodiment has a Charpy impact value of 20 J/cm² or more and excellent cold cracking resistance. Therefore, the cold cracking does not occur even when continuous annealing or a pickling step is carried out after hot rolling. Accordingly, the hot-rolled ferritic stainless steel sheet of the present embodiment can be appropriately applied to a step of manufacturing members such as household electric appliances, building materials, and automobile components for which the ferritic stainless steel is used.

ti 

1. A hot-rolled ferritic stainless steel sheet with excellent cold cracking resistance, comprising, in terms of % by mass: 0.0150% or less of C; 0.01% to 2.00% of Si; 0.01% to 2.00% of Mn; less than 0.040% of P; 0.010% or less of S; 10.0% to 30.0% of Cr; 0.001% to 3.00% ofAl; and 0.0200% or less of N, with a balance being Fe and unavoidable impurities, wherein in a cross section in a range of ¼ to ¾ of a sheet thickness, a length L of all crystal grain boundaries having orientation differences of 1° or more to less than 180° and a length La of subgrain boundaries having orientation differences of 1° or more to less than 15° satisfy a relation of La/L≧0.20.
 2. The hot-rolled ferritic stainless steel sheet with excellent cold cracking resistance according to claim 1, which further comprises one or more selected from, in terms of % by mass: 0.05% to 0.70% of Nb; 0.05% to 0.30% of Ti; 0.1% to 2.5% of Mo; 0.1% to 1.5% of Ni; 0.0001% to 0.0025% of B; 0.1% to 2.0% of Cu; and 0.03% to 0.35% of Sn, wherein in a case where either one or both of Nb and Ti are included, the following formula (1) is satisfied, Nb/93+Ti/48≧C/12+N/14   (1) element symbols in the formula (1) indicate contents of the respective elements in terms of % by mass.
 3. The hot-rolled ferritic stainless steel sheet with excellent cold cracking resistance according to claim 1 or 2, wherein the content of Al is in a range of more than 0.10% to 3.00%.
 4. A method for manufacturing the hot-rolled ferritic stainless steel sheet with excellent cold cracking resistance according to any one of claims 1 to 3, the method comprising: casting ferritic stainless steel having the steel composition according to any one of claims 1 to 3 to generate a semi-finished product and subjecting the semi-finished product to hot rolling under a condition where a finishing temperature is in a range of 800° C. to 1000° C. to generate a hot-rolled steel sheet; subsequently, coiling the hot-rolled steel sheet in a coil shape at a temperature of more than 650° C. to 800° C.; and immersing the hot-rolled sheet coiled in a coil shape in a water tank within 1 hour after the coiling, maintaining the hot-rolled sheet in the water tank for 1 hour or more, and taking out the hot-rolled steel sheet. 